In recent years, in communication, broadcasting, and even with recording information on or reproducing information from an optical disk or a magnetic disk, recording/reproducing devices for handling soft decision values obtained by calculating reliability information are being proposed in order to utilize a turbo code or an LDPC code (a low-density parity check code) which are known to produce error correcting performances that approach theoretical limitations (for example, refer to Patent Literature 1).
FIG. 8 is a diagram showing a configuration of a conventional recording/reproducing device which handles soft decision values. In FIG. 8, a recording/reproducing device 101 comprises an encoding unit 102, a PR (Partial Response) communication path 103, and a decoding unit 104.
The encoding unit 102 comprises an error correction encoding unit 111 and a modulation encoding unit 112. The error correction encoding unit 111 generates an error correction encoded sequence by adding a parity sequence based on a predetermined rule to an inputted user data sequence. The modulation encoding unit 112 encodes the inputted error correction encoded sequence to a predetermined modulation code based on a predetermined modulation rule, and outputs the modulation code to which a predetermined constraint has been added as an encoded sequence to the PR communication path 103.
Moreover, examples of a constraint that is used as the predetermined constraint include a DC free constraint in which the numbers of “0”s and “1”s in a code are equalized over a sufficiently long range and a (d, k) constraint in which a minimum length and a maximum length of the number of consecutive “0”s respectively take values of d and k.
The PR communication path 103 comprises a recording/reproducing unit 113 and an equalization processing unit 114. For example, the PR communication path 103 performs a recording process or a reproducing process in a PR2 (Partial Response class-2) recording/reproducing channel.
The recording/reproducing unit 113 performs NRZI (non return to zero Inverted) encoding on the encoded sequence inputted from the modulation encoding unit 112, and records the NRZI-encoded signal on a mounted recording medium or a built-in recording medium using a mark edge recording method.
In addition, the recording/reproducing unit 113 reads out an encoded signal recorded on the recording medium in the PR2 channel and supplies the read encoded signal to the equalization processing unit 114.
The equalization processing unit 114 performs a PR (Partial Response) equalization process that utilizes a waveform interference on the encoded signal supplied from the recording/reproducing unit 113 so that predetermined target equalization characteristics are produced, and supplies the encoded signal to the decoding unit 104.
The decoding unit 104 comprises a PR-SISO decoding unit 115, a SISO demodulating unit 116, and an error correction decoding unit 117. The PR-SISO decoding unit 115 performs a predetermined decoding process on the encoded signal supplied from the equalization processing unit 114 and outputs a soft decision value.
In this case, SISO stands for Soft-Input Soft-Output which refers to a process for inputting and outputting a soft decision value.
The PR-SISO decoding unit 115 obtains a trellis expression that is an expansion, along a time series, of a state transition chart which represents an encoding process at each time based on the NRZI encoding and the PR2 channel from the encoded signal from the PR communication path 103, performs a probability operation based on the obtained trellis expression, and calculates reliability information as a soft decision value.
The SISO demodulating unit 116 calculates reliability information that has been modulation-decoded using a trellis based on the modulation rule of the modulation encoding unit 112.
For example, the calculation of reliability information using a trellis by the PR-SISO decoding unit 115 and the SISO demodulating unit 116 is executed by a BCJR (Bahl-Cocke-Jeinek-Raviv) algorithm.
For example, the error correction decoding unit 117 uses turbo decoding. The error correction decoding unit 117 executes error correction by performing turbo decoding that corresponds to the turbo code used by the error correction encoding unit 111. Moreover, Patent Literature 1 also presents an example of error correction decoding performed using an LDPC code.
Moreover, hereinafter, unless otherwise noted, reliability information refers to a logarithmic ratio of a decoding probability of a symbol or a minimum unit of information. For example, the minimum unit of information may be a bit that represents a binary code or, in other words, “0” and “1” or may be a multilevel element.
In addition, Patent Literature 2 describes a Sum-Product algorithm that realizes decoding by an LDPC code.
Furthermore, Non Patent Literature 1 describes an embodiment of soft decision decoding by a trellis that is based on a PR communication path and a modulation rule when processing a signal on an optical disk.
However, with a conventional decoding system based on a modulation rule that limits an appearance pattern of symbols of recorded information on an optical disk or a magnetic disk, the reliability information that is calculated by the SISO demodulating unit 116 shown in FIG. 8 may not be correlated with a noise distribution of a communication path. For example, there may be cases where a bias in distribution of reliability information occurs such as when reliability information is distributed over a wide range.
In addition, even in the field of optical communication, since a band of a transmission signal is limited according to characteristics of an optical fiber, a modulation rule is conceivable which limits an appearance pattern of symbols in transmitted information. For example, there may be cases where a bias in distribution of reliability information occurs such as when reliability information is distributed over a wide range.
Such a distribution bias may include an error bit with high reliability. This may occur when, for example, a given modulation pattern is demodulated as another modulation pattern and the given modulation pattern is SISO-demodulated as a modulation pattern with high reliability even though the given modulation pattern includes an error.
When error correction is performed on reliability information including an error bit with high reliability, the reliability information cannot be stochastically properly error-corrected. As a result, an error floor may be created in which an error occurs even in an ideal environment where an error probability of a communication path is sufficiently low.